The present invention relates to methods and apparatuses for power management in photovoltaic (PV) installations. FIG. 1 is a high-level circuit diagram of a typical related-art PV installation 10. As shown in FIG. 1, the PV installation 10 includes a plurality of PV modules 20 connected in series and in parallel. The PV modules 20 in each column are connected in series, and the columns of PV modules 20 are connected in parallel to form the PV installation 10. For example, a column may include approximately 20 PV modules 20, and the PV installation 10 may include approximately 250 columns. As shown in FIG. 1, typical PV installations use a single centralized inverter 30 to convert the DC output of the PV installation 10 to AC power to be provided to the grid 40. A large-scale grid-connected PV installation 10 may generate anywhere between 500 W and 5 kW for residential systems to over 1 MW for PV power plant applications.
For PV power plant applications, the input DC voltage of the centralized inverter 30 is typically around 500 V, and the output AC voltage may be on the order of 120 V up to 100 kV. The centralized inverter 30 shapes the AC output voltage or current to obtain a clean 60 Hz sinusoidal waveform with strictly regulated noise and distortion limits. The centralized inverter 30 also manages the DC voltage across the PV installation 10. There is typically an optimum DC voltage for the PV installation 10 known as the maximum power voltage (MPV). This is the DC voltage that coincides with the maximum power point (MPP) of the PV installation 10, which is the operating point where the PV installation 10 produces the maximum power. The MPV also coincides with a maximum power current (MPC), which is the current that coincides with the MPP. The centralized inverter 30 can typically regulate the power flowing out of the PV installation 10 to drive the PV installation 10 to the MPP with a maximum power point tracking (MPPT) algorithm.
With a single centralized inverter 30 managing the entire PV installation 10, all of the columns of PV modules 20 within the PV installation 10 operate with the same voltage, because they are connected in parallel. This can be a problem if the MPVs of the PV modules 20 are different. Additionally, within the columns, each of the PV modules 20 operates with the same series current, because they are connected in series. This can be a problem if the MPCs of the PV modules 20 are different. Although each PV module 20 is designed to have the same characteristics, the MPP, MPV, and MPC of a PV module 20 can change if the PV module 20 encounters local shading, dirt, or debris. Further, there can be a factory mismatch between two PV modules 20, in which the characteristics of the two PV modules 20 are slightly different as manufactured.
The PV module 20 in the lower left-hand corner of the PV installation 10 is shown in FIG. 1 as being partially shaded. This shading causes shifts in the MPV and MPC of the shaded PV module 20 such that the PV module 20 may be forced to a sub-optimal operating point. In some cases, the PV module 20 may contribute nearly zero power, even with only small amounts of partial shading. For example, if only 20% of the incident light is blocked from reaching the PV module 20, the characteristics of the shaded PV module can change substantially relative to the un-shaded PV modules 20.
Because the PV modules 20 are connected in series, they must have the same operating current. If the shaded PV module 20 is forced to the MPC of the un-shaded PV modules but the shaded PV module 20 does not have sufficient current available to support that MPC, then the voltage of the shaded PV module 20 will collapse and the shaded PV module 20 will contribute zero power. Alternatively, if the unshaded PV modules 20 are forced to the MPC of the shaded PV module 20, the unshaded PV modules 20 will operate at a current below their optimal MPC, and contribute less than the maximum power available. In this case, many of the unshaded PV modules 20 may operate sub-optimally because of the shading of one PV module 20. In both cases, the PV installation 10 operates sub-optimally because the centralized inverter 30 regulates many PV modules in aggregate, although the individual PV modules 20 may have varied characteristics. Accordingly, an apparatus and method are needed to address the problems caused by shading of PV modules 20 in the PV installation 10.